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9/8/98 AC 43.13-1B SECTION 4. MAGNETIC PARTICLE INSPECTION 5-40. GENERAL. Magnetic particle in- spection is a method for detecting cracks, laps, seams, voids, pits, subsurface holes, and other surface, or slightly subsurface, discontinuities in ferro-magnetic materials. Magnetic particle inspection can be used only on ferro-magnetic materials (iron and steel). It can be performed on raw material, billets, finished and semi- finished materials, welds, and in-service as- sembled or disassembled parts. Magnetic par- ticles are applied over a surface either dry, as a powder, or wet, as particles in a liquid carrier such as oil or water. Common uses for magnetic particle inspection are; final inspection, receiving inspection, in- process inspection; and quality control, main- tenance, and overhaul. 5-41. PRINCIPLES OF OPERATION. Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an ap- plied field to pass through the metal rather than through the air. A defect at or near the metal’s surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface. (See figure 5-9.) The field strength is increased in the area of the defect and opposite magnetic poles form on either side of the defect. Fine magnetic particles applied to the part are attracted to these regions and form a pattern around the de- fect. The pattern of particles provides a visual indication of a defect. (See figure 5-10.) FIGURE 5-9. Magnetic field disrupted. FIGURE 5-10. Crack detection by magnetic particle in- spection. a. To locate a defect, it is necessary to control the direction of magnetization, and flux lines must be perpendicular to the longitudinal axes of expected defects. Examination of critical areas for defects may require complete disassembly. Two methods of magnetization, circular and longitudinal, are used to magnet- ize the part and induce perpendicular flux paths. Parts of complex configuration may re- quire local magnetization to ensure proper magnetic field direction and adequate removal of surface coatings, sealants, and other similar compounds. Possible adverse influence of the applied or residual magnetic fields on delicate parts such as instruments, bearings, and mechanisms may require removal of these parts before performing the inspection. b. Certain characteristics inherent in the magnetic particle method may introduce errors in examination results. Nonrelevant er- rors are caused by magnetic field distortions due to intentional design features, such as: (1) Sharp radii, less than 0.10 inch ra- dius, in fillets; (2) Thread roots, keyways, and drilled holes; and (3) Abrupt changes in geometry or in magnetic properties within the part. Par 5-40 Page 5-19PDF Image | AFS-640
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